Effect of biaxial and triaxial stresses on coal mine shale rocks

High horizontal in-situ stresses and weak immediate roof rock have been a detrimental combination for underground coal mines in the eastern United States. Also, these conditions pose challenging ground control problems to researchers. One such problem, referred to by many names, is the "cutter roof," "kink failure," "gutter," or "pressure cutting." It is a compressional type failure of the roof, commonly observed in Appalachian underground coal mines with laminated roof rock.;The available literature concerning the aforementioned failure is limited in terms of our understanding of the mechanism underlying the failure. The fundamental research presented in this thesis is an attempt to provide a deeper understanding of the mode and/or mechanism underlying laminated rock failure. For this work, a special platen was fabricated and is capable of applying biaxial compressive stress on a specimen when the entire arrangement is used inside a uniaxial compressive loading device. The experimental set-up was further modified to apply a pseudo-triaxial compressive stress on the specimen.;Delamination of the laminated shale specimens was observed in the biaxial stress condition. Close monitoring of these tests revealed that the inner layers moved outward while the outer layers moved toward the unconfined area and warped before they failed. Limestone specimens tested under similar condition showed a different mode of failure. This test also showed that the failure mechanism observed in the laminated shale was unique and not an artifact of the experimental design. Lastly, the influence of lamination on the failure mode was observed to be less under triaxial stress conditions. Laminated shale specimens failed along conjugate shear planes along with compaction of rock under confinement.